1.Field of the Invention:
The present invention relates to a device for optically measuring the shape of an aspheric surface.
2. DESCRIPTION OF THE PRIOR ART
It has been general practice to optically measure the aspheric shape of an object surface by applying a spherical wave of light to the object surface to be measured, measuring the shape of a wavefront of light reflected from the object surface, and correcting the measured wavefront shape with the shape of the incident spheric wave.
Where only one spherical wave is available for application to an aspheric surface to be measured, the aspheric surface must be of a shape analogous to that of the applied spherical wave. If the shape of the aspheric surface to be measured deviated largely from the shape of the applied spherical wave, then the shape of the aspheric surface could not be measured. Therefore, in case only one spherical wave is employed for measurement, the measurement range for aspheric surfaces is relatively small.
One proposed way of widening the measurement range for aspheric surfaces is to use a plurality of spherical waves having different radii of curvature in measuring one aspheric surface (see "Method of measuring an aspheric surface shape using a plurality of interferograms" in Optics, Vol. 12, No. 4, Aug. 1983). This method is based on the Twyman-Green interferometer. However, the proposed process is disadvantageous in that extraneous light tends to enter a region being measured, causing a measurement error, and the measurement accuracy cannot be increased since the resolution for reading interference fringes is low.